They are with sponge - awesome sound proof compared to regular headphones...
What to look for? size - shape ...

I was thinking of making my own headphones next time
Have a few plastic piezo speakers and noise cancellation headphones.

My biggest issue with my ML blue headphones is that sometimes one side turns away from my head and its guides
and when I snorkel takes time to fill up with water and I don't hear sounds in that time. (and I dive 3-4x to get a target)
I have my snorkel on left side - headphones are in the way ... my jaw / teeth hurts from the pressure on the snorkel.